MONOCLINIC PHASE OF BORON-NITRIDE APPEARING DURING THE HEXAGONAL CUBIC PHASE-TRANSITION AT HIGH-PRESSURE AND HIGH-TEMPERATURE

Fine structures appearing on the phase transition from h (hexagonal) t o c (cubic) boron nitride under high pressure (7.7 GPa) and high tempe rature (1800-2150 degrees C) are examined by high-resolution transmiss ion electron microscopy. A prominent contraction of the interplanar sp acing between sp(2) sheets from 3.33 to 3.10 Angstrom in so-called ''c ompressed h-BN'' is attributable to a monoclinic lattice distortion of the residual h-BN, which originates from the difference in the compre ssibility as well as the thermal expansion between adjoining h- and c- BN grains. The parameters of the monoclinic unit cell are a(m) = 4.33, b(m) = 2.50, c(m) = 3.1-3.3 Angstrom, and beta = 92-95 degrees. Thin plates of h-BN are often folded and the folding also causes the monocl inic structure. The sheet sequence of I (rhombohedral)-BN locally appe ars when the strong volume shrinkage occurs due to the formation of a c-BN grain. Nanoscale twins appear in resulting c-BN grains, as long a s they are small, and tv(wurzite)-BN is sometimes included in them. (C ) 1996 American Institute of Physics.